Retention of Egg Yolk Proteins Leads to New Understanding of Viviparity in Sharks and Related Fishes

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In a recent study, a team of researchers led by Shigehiro Kuraku uncovered a new function of egg yolk proteins in viviparous sharks and their relatives. Their results suggest that these proteins may have been retained in cartilaginous fishes during the evolution of viviparity and may play a role in providing nutrition to the developing embryo.


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Viviparity, or the ability to give birth to live young, is commonly linked to mammals. However, this mode of reproduction has evolved multiple times across various vertebrates, with over 150 separate occurrences. This includes over 100 instances in reptiles, 13 in bony fishes, 9 in cartilaginous fishes, 8 in amphibians, and 1 in mammals.

Consequently, to comprehend the evolution of viviparity, it is necessary to investigate the trait across different evolutionary lineages. In the case of cartilaginous fishes, which include species such as sharks, skates, and rays, as many as 70% of these species give birth to live young. Despite this, the viviparity in these animals remains inadequately understood due to their elusive nature, low reproductive output, and large and repetitive genomes.

Sharks and their relatives are the only group among fishes with more than one form of reproduction.

In a recent article published in Genome Biology and Evolution, a team of researchers led by Shigehiro Kuraku, previously Team Leader at the Laboratory for Phyloinformatics at RIKEN Center for Biosystems Dynamics Research in Japan, set out to address this gap. Their study identified egg yolk proteins that were lost in mammals after the switch to viviparity but retained in viviparous sharks and rays. Their results suggest that these proteins may have evolved a new role in providing nutrition to the developing embryo in cartilaginous fishes.

More than 70% of cartilaginous fishes give birth to live young.

According to Kuraku, who now works as a Professor of Molecular Life History Laboratory at the National Institute of Genetics in Mishima, investigators have long wanted to learn more about the evolution of viviparity in sharks and their relatives. "Reproduction is one of the most fascinating features of cartilaginous fishes because they show a broad spectrum of reproductive modes." .

Among viviparous species, this includes a range of mechanisms for providing nutrients to the developing embryo, from relying solely on nutrients present in the embryo’s yolk sac, to feeding the embryo unfertilized eggs, secreting nutrients from the uterus ("uterine milk"), or transferring nutrients via a placenta.

Viviparous sharks and their relatives rely on nutrition from the mother, which is either supplied from their yolk sac or directly via a placenta.

To better understand these various mechanisms, the authors searched genomic and transcriptomic data from 12 cartilaginous fishes for homologs of vitellogenin (VTG), a major egg yolk protein synthesized in the female liver in egg-laying species. Regardless of their reproductive mode, all cartilaginous fish species had at least two copies of VTG, while all copies of VTG have been lost from mammals (although the authors did identify a copy in the Tasmanian devil, a marsupial, which was not previously known to harbor a VTG gene).

Egg yolk proteins have been conserved in cartilaginous fishes, with three copies of the egg yolk protein receptor present in the genomes of these species.

Next, the authors searched for homologs of the VTG receptor; while mammals retain a single copy of this receptor, Kuraku and his colleagues identified two ancient tandem duplications giving rise to three copies of the receptor in cartilaginous fishes. The authors note that this finding was unexpected.

"We predicted the retention of egg yolk protein genes in the shark genomes because live-bearing sharks rely partly on nutrition supply from the egg yolk," says Kuraku. "What surprised us the most was that cartilaginous fish including sharks have more copies of the egg yolk protein receptor genes." .

The expression patterns of egg yolk proteins in cartilaginous fish genomes suggest that these proteins may be involved in transporting nutrition from the mother to the developing embryo.

This suggested that these proteins may provide a novel function in this viviparous lineage.

To shed light on the functions of VTG and its recepters in the cartilaginous fishes, the authors sought to compare the gene expression of these proteins between egg-laying as well as live-bearing species. In cartilaginous fishes, the expression pattern of both VTG and its receptor genes were correlated with the presence of viviparity, suggesting that these proteins may be involved in transporting nutrition from the mother to the developing embryo.

The Tasmanian devil was recently discovered to have an egg yolk protein gene that has been lost in other mammals

In summary, Kuraku and his team uncovered a novel function of egg yolk proteins in viviparous sharks and their relatives. Their results suggest that these proteins may have been retained in cartilaginous fishes during the evolution of viviparity and may play a role in providing nutrition to the developing embryo.


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